A pivoted mirror reflects a beam of light to a photosensitive plate or paper. Changes in pressure in the engine cylinder cause the mirror to oscillate up and down; a pin and eccentric actuated by a flexible shaft connected to the engine shaft cause the mirror to oscillate from side to side.
The indicator is housed in a wooden box that is 5 inches by 6 inches by 14 inches, with a brass pipe fitting and gear box at one end. The opposite end is open to receive a plate holder of a ground glass. A T-shaped tube lets light into one side, and an adjustable prism inside directs the light to the mirror. The flexible shaft is 32 inches long and covered with black fabric. Four plate holders, one ground glass, a steel petcock and a tapered hollow steel plug are included with the instrument. It is marked “Manographe Hospitalier-Carpentier, Brevete, S G D G, J. Carpentier, Paris.”
An engine indicator is an instrument for graphically recording the pressure versus piston displacement through an engine stroke cycle. Engineers use the resulting diagram to check the design and performance of the engine.
A mechanical indicator consists of a piston, spring, stylus, and recording system. The gas pressure of the cylinder deflects the piston and pushes against the spring, creating a linear relationship between the gas pressure and the deflection of the piston against the spring. The deflection is recorded by the stylus on a rotating drum that is connected to the piston. Most indicators incorporate a mechanical linkage to amplify the movement of the piston to increase the scale of the record.
When the ratio of the frequency of the pressure variation to the natural frequency of the system is small, then the dynamic deflection is equal to the static deflection. To design a system with a high natural frequency, the mass of the piston, spring, stylus, and mechanical linkage must be small, but the stiffness of the spring must be high. The indicator is subjected to high temperatures and pressures and rapid oscillations, imposing a limitation on the reduction in mass. Too stiff a spring will result in a small displacement of the indicator piston and a record too small to measure with accuracy. Multiplication of the displacement will introduce mechanical ad dynamic errors.
The parameters of the problem for designing an accurate and trouble free recorder are such that there is no easy or simple solution. Studying the variety of indicators in the collection shows how different inventors made different compromises in their designs.